The present invention relates to a rotating-drum scanning apparatus with a scanning head including a plurality of scanning elements.
Rotating-drum scanning apparatuses are conventionally known in which an original copy is wound around a rotating drum and information on the original is read by a photoelectric sensor, or in which a recording sheet is wound around the rotating drum and an image is recorded on the recording sheet by means of a recording head including recording elements, such as an ink-jet nozzle, electric discharge recording stylus, electrostatic recording stylus, heat generating element, etc.
In general, a rotating-drum scanning apparatus comprises a rotating drum, a guide shaft extending parallel to the axis of the rotating drum, and a scanning head supported by the guide shaft and movable parallel to the axis of the drum, the scanning head including one scanning element. A rectangular original or recording sheet as an object of scanning is wound around the outer peripheral surface of the rotating drum so that its one side extends parallel to the axis of the drum. In this state, the rotating drum is rotated at a fixed speed so that the object on the rotating drum is subjected to primary scanning in the circumferential direction of the drum by the scanning head. As the scanning head is moved in one direction at a fixed speed, synchronized with the rotation of the drum, moreover, the object is subjected to secondary scanning in the axial direction of the drum by the scanning head. As a result, a scanning line substantially perpendicular to the axis of the drum is formed on the object by the scanning element every time the rotating drum makes one revolution.
In the scanning apparatus constructed in this manner, the object on the drum is scanned at high speed by means of a multi-type scanning head which includes a plurality of scanning elements arranged in a straight line in the secondary-scanning direction. If a plurality of, e.g., five, scanning elements are arranged at predetermined pitches in the secondary-scanning direction, the scanning head is moved in the secondary-scanning direction over a distance about five times as long as the distance for a scanning head with a single scanning element while the rotating drum makes one revolution. If scanning is performed in synchronism with the rotation of the rotating drum, therefore, each scanning element spirally scans the outer peripheral surface of the drum, forming a scanning line at an angle to a direction perpendicular to the axis of the drum. If the circumference of the drum is L and the distance covered by the scanning head for each revolution of the rotating drum is .DELTA.L, the tilt angle .alpha. of the scanning line with respect to the direction perpendicular to the axis of the drum is given by EQU .alpha.=tan.sup.-1 .DELTA.L/L . (1)
If a scanning head with a single scanning element is used for scanning, the distance .DELTA.L is about 0.05 to 0.2 mm, the length L is about 200 to 300 mm, and the tilt angle .alpha. is negligible. If a scanning head with a plurality of, e.g., ten, scanning elements are used, on the other hand, the distance .DELTA.L is about 1 mm, and the tilt angle .alpha. of the scanning lines is too large to be ignored. It is very difficult to remove the tilt of the scanning lines in the primary-scanning direction by electrical signal processing.
To prevent the tilting of the scanning lines, a method is proposed in which the scanning head is stopped during primary-scanning cycles so that it is intermittently moved for secondary scanning, taking advantage of the intervals between the primary-scanning cycles. In this case, however, the primary and secondary scanning operations cannot simultaneously be performed, so that the total scanning time is long. Moreover, the scanning head needs to be accurately intermittently moved, requiring a sophisticated, expensive drive mechanism.
When using a plurality of scanning heads for color-print recording, the scanning heads are arranged along the circumference of the rotating drum. In this case, the individual scanning heads are moved with staggered timing, so that they require independent drive units of their own, resulting in a complicated and expensive drive mechanism. If a plurality of scanning heads are arranged in the secondary-scanning direction, furthermore, the scanning operation is time-consuming as a whole.